The present invention relates to an improved image reader.
An image reader produces a picture signal by converting light from an object into an electrical signal for each picture element by means of a photoelectric conversion device. However, the picture signal thus produced includes a distortion component (shading) caused by non-uniformity of the optical strength from a light source, variation of the optical strength-distribution from an optical system and differences of sensitivity between elements constituting the photoelectric conversion device.
An image reader in which a photoelectrically converted picture signal is converted into a digital siganl by an A/D (analog-to-digital) converter has adopted a method for eliminating the shading as follows.
First, light reflected at a standard white board is photoelectrically converted into an analog signal. The analog signal is converted into a digital signal to be stored in a compensation memory prior to image-reading.
Then, a picture signal is produced by actual image-reading, digitized and multiplied by the signal stored previously in the compensation memory corresponding outputs from each element or picture element of the image reader to compensate for the shading.
This system requires a digital multiplier for shading-compensation. Furthermore, when the number of quantized bits is large, the number of digits required for calculation increases greatly. This results in the circuit configuration of the system becoming complex.
Moreover, the photoelectric conversion device has a problem for the accuracy of analog-to-digital conversion due to the dark current if a picture signal with a large gradation is to be produced or if the number of quantized bits is to be made large.